1. Field of the Invention
This invention relates to means to purify municipal or industrial wastewater.
2. State of the Art
In a conventional wastewater treatment system of the activated sludge type, particle-bearing wastewater is passed through a primary clarifier wherein solid particles settle gravitationally while clarified wastewater is removed from the upper part of the clarifier. The clarified wastewater is thereafter passed into a biological treatment unit wherein air or oxygen is added to encourage the growth and reproduction of micro-organisms in the wastewater. The effluent from the biological treatment unit is then passed into a so-called secondary clarifier wherein the micro-organisms settle to the bottom while clarified wastewater is removed and discharged to a receiving water such as a river. Concomitantly, the settled micro-organisms in the secondary clarifier are removed and portions thereof are returned to the biological treatment unit to provide therein a living culture of micro-organisms called activated sludge. The remainder of the micro-organisms settled in the secondary clarifier (called secondary sludge) and the solid particles settled in the primary clarifier (called primary sludge) are together discharged from the system for disposal. The primary and secondary sludges usually contain about 97% water or more; usually it is desirable to remove much of that water to increase the concentration of solids and to reduce the cost of subsequent disposal of the sludge. Devices which remove water from sludge are called thickeners and may be of various kinds, including gravity and flotation type.
In a typical gravity type thickener, sludge is allowed to settle in a tank so that sludge particles become more concentrated near the bottom of the tank while clarified liquid rises to the surface as it is displaced by the settling particles. Thickened sludge is then removed from the bottom of the tank.
In a typical flotation type thickener, feed sludge is first aerated at super atmospheric pressure and then is introduced into the flotation tank, which is at atmospheric pressure, so that the air within the sludge forms into bubbles which carry sludge particles to the liquid surface in the tank. The thickened sludge at the tank surface is then removed while clarified liquid is removed from the lower part of the tank.
It is generally agreed that flotation type thickeners are superior to other types for thickening secondary sludge and, on the other hand, that gravity type thickeners are superior for thickening primary sludge. It is also known that primary sludge in a gravity type thickener often is decomposed by micro-organisms which do not require oxygen to sustain their metabolic processes. Such anaerobic decomposition is normally considered undesirable because it results in the production of gases which have unpleasant odors and which inhibit effective thickening. It has been suggested to reduce anaerobic decomposition during sludge thickening by covering the sludge with a blanket of liquid containing air; such a process is taught, for example, in U.S. Pat. No. 3,140,259.